Damped intershaft bearing and stabilizer

ABSTRACT

A viscous damper for a bearing mounted between coaxially mounted shafts rotating at relative speeds is stabilized by mounting a spring in parallel with the damper so as to raise the natural frequency of one of the shafts.

BACKGROUND OF THE INVENTION

This invention relates to intershaft bearings of the type utilized forsupporting coaxially mounted shafts interconnecting the compressors andturbines of a multi-spool gas turbine engine and particularly to meansfor stabilizing the viscous damper.

As is well known in the art an inherent vibratory instability can existin a multi-shaft rotor system which is connected by a viscous (oil)damper. This instability is similar to the well known "oil whip" whichis experienced on high speed rotating machinery which incorporates plainjournal bearings. The instability results from the development of arotating pressure field when one shaft spins. This pressure can excitethe natural frequency of the other shaft and force the system into high,uncontrollable vibration.

We have found that we can obviate the problem noted above and eliminatethis instability from the operating speed range of the first shaft byraising the natural frequency of the second shaft with a spring mountedin parallel with the viscous damper. This spring can either be a metalstructure or an elastomeric one. Thus, in accordance with this inventionincorporated between two rotating shafts the damper can be made toperform properly without producing unstable, high amplitude response.

SUMMARY OF THE INVENTION

An object of this invention is to provide an improved viscous damper foran intershaft bearing by incorporating a parallelly mounted spring whichstabilizes said viscous damper.

A further object of this invention is to provide parallel mountedsprings to a viscous damper for an intershaft bearing utilized tosupport the coaxially mounted shafts of a gas turbine engine which ischaracterized by being simple to implement and relatively inexpensiveparticularly when compared to the alternative of a heavier and largershaft and/or additional support bearings and/or viscous dampers at otherbearing locations. The springs contemplated may be either metal orelastomeric.

Other features and advantages will be apparent from the specificationand claims and from the accompanying drawings which illustrate anembodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial view in sectional showing a preferred embodiment ofthis invention.

FIG. 2 is another partial view in section showing another embodiment.

FIG. 3 is a perspective view showing the spring utilized in the FIG. 1embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of this invention only that portion of the bearing andshafts of a turbine power plant is shown that is necessary for anunderstanding of this invention. As shown in FIG. 1 the coaxiallymounted hollow shafts 10 and 12, (mounted about center line 14) areshown at least partially supported by bearing 16. Bearing 16 includes anouter ring 18 defining an outer race and an inner ring 20 defining aninner race supporting therebetween a plurality of circumferentiallymounted roller elements 22 (only one being shown). Sleeve element 24firmly affixed to shaft 12 and rotatable therewith supports bearingretainer 26 and also rotates therewith. Suitable locking nuts 28 and 30are suitably secured to the threaded end of shaft 10 and threaded end ofretainer 26.

An annular groove 32 is formed at one end of sleeve element 24 and liesin the same plane of rollers 22 for defining a fluid or viscous damper.Piston rings 34 and 36 or other suitable sealing elements fitted intorecesses adjacent the annular groove 32 serve to prevent the fluid whichcompletely fills this space from leaking. An inlet and outlet, notshown, assures that the damper has a flow of oil under pressure. Dampersas described are well known and do not form the essence of thisinvention. The purpose of this invention is to be able to utilize suchdampers and allow them to function properly in a high speed intershaftapplication.

In accordance with this invention a spring 40 flexible in a radialdirection and stiff axially is mounted parallel to damper 32 by havingone end firmly affixed to shoulder 42 and the other end 44 firmlyaffixed to the end of bearing retainer 26. Spring 40 may be a metalhollow cylinder having axial slots 46 cut along the circumference topermit flexibility and resiliency. The geometry is selected to provide aspecific stiffness between shafts 10 and 12 which will raise the naturalfrequency of shaft .[.12.]. .Iadd.10 .Iaddend.above the operating speedof shaft .[.10.]. .Iadd.12.Iaddend.. In doing so, the tendency of havingthe natural frequency of this shaft become excited is eliminatedpermitting the damper to function properly without producing unstable,high amplitude response.

FIG. 2 is substantially identical to the embodiment of FIG. 1 and likeelements designate like parts. Similarly, viscous damper 32 is mountedin the plane of roller element 22. The bearing retainer 26 is recessedat either end to accommodate the ring elements 50 and 52, which arerectangular in cross section. The material selected for ring elements 50and 52 is a well known commercially available elastomeric substanceexhibiting good spring characteristics and capable of withstanding thetemperature environment. Like spring 40, elements 50 and 52 are parallelwith respect to damper 32 and serve as a spring in the same manner asdoes spring 40. It is apparent from the foregoing that in bothembodiments described springs 50 and 52 and spring 40 serve to changethe critical speed of shaft 10 and in so doing permits the damper, whichheretofore did not function at high speeds, to operate successfully.

It is contemplated within the scope of this invention that the outershaft can be the supporting shaft rather than the inner shaft as shown.In this instance, the viscous damper annulus would surround the bearingouter race and the spring would be mounted in parallel therewith. Also,as one skilled in this art will appreciate the invention is not limitedto any particular bearing or bearing configuration, as ball, tapered,fluid bearings and the like could equally be employed.

It should be understood that the invention is not limited to theparticular embodiments shown and described herein, but that variouschanges and modifications may be made without departing from the spiritor scope of this novel concept as defined by the following claims.

We claim:
 1. Damping means for an inner shaft and outer shaft mounted incoaxial relation, bearing means mounted between said inner shaft andsaid outer shaft, a fluid damper mounted under the bearing and includesan annular groove surrounding at least one of said shafts which isfilled with a viscous fluid, radial restraint means mounted on said oneof said shafts in parallel with the viscous damper means to impart aload to the other of said shafts to change its critical speed to a valueabove the maximum rotational speed of said one of said shafts andstabilize the viscous damper.
 2. Damping means as claimed in claim 1wherein said radial restraint means includes ring-like elements spacedon opposite ends of said annular groove and are sandwiched between saidbearing means and said one of said shafts.
 3. Damping means as claimedin claim 2 wherein said ring-like elements are made from an elastomericmaterial.
 4. Damping means as claimed in claim 2 wherein said ring-likeelements are made from a metallic material.
 5. Damping means as claimedin claim 1 wherein said radial restraint means includes a cylinder-likeelement surrounding said one of said shafts having one end attached tosaid one of said shafts and the other end attached to said bearing meansforming a cantilever spring.
 6. In combination, inner and outercoaxially mounted shafts for a twin spool gas turbine power plant, aroller bearing between said shafts including an inner race adjacent saidinner shaft, a race supporting ring secured to said inner shaft torotate therewith supporting said inner race, a viscous damper includingpressurized viscous fluid in an annular space between said shaft andsaid race supporting ring, axial extension portions on said racesupporting ring radially spaced from said inner shaft, spring meansmounted in said space to add stiffness in parallel with said viscousdamper through said bearing means to said outer shaft to increase itscritical speed relative to said inner shaft maximum speed whereby saidviscous damper is rendered stable.
 7. In combination as in claim 6wherein said spring means includes an elastomeric ring-like elementflexible radially.
 8. In combination as in claim 6 wherein said springmeans includes a metallic spring-like element flexible radially.
 9. Incombination, inner and outer coaxially mounted shafts for a twin spoolgas turbine power plant, a roller bearing between said shafts includingan inner race adjacent said inner shaft, a race supporting ring securedto said inner shaft to rotate therewith supporting said inner race, aviscous damper including pressurized viscous fluid in an annular spacebetween said shaft and said race supporting ring, a sleeve secured tosaid inner shaft to rotate therewith between said race supporting ringand inner shaft, spring means interconnecting said sleeve and said racesupporting ring to add stiffness in parallel with said viscous damperthrough said bearing means to said outer shaft to increase its criticalspeed relative to said inner shaft whereby said viscous bearing isrendered stable.
 10. In combination as in claim 9 where said sleeveincludes a flange axially spaced from said race supporting ring and oneend of said spring means is secured to said flange and its opposing endis secured to said race supporting ring.
 11. In combination as in claim10 wherein said spring means includes a cylindrical member havinghub-like opposing ends with one end secured to said flange and the otherend secured to said race supporting ring, and rod-like elementsextending between said hub-like opposing ends deflectable in a radialdirection.